Brucellosis is a one of the most widespread zoonotic infection, with over 500,000 new cases each year. The disease, caused primarily by the bacterial pathogens Brucella melitensis and Brucella abortus, is acquired by consumption of unpasteurized dairy products or by contact with infected animals. Brucellosis is a multisystem disease characterized by fever, and frequently, osteoarticular disease. Since it has been estimated that only 4% of brucellosis cases are reported, the actual number is likely to be much larger. An important feature of the disease is its protracted nature that results from persistent bacterial colonization of the reticuloendothelial system and systemic dissemination. However, the specific interactions between pathogen and host that result in successful colonization and evasion of clearance by the immune system are largely uncharacterized. While Brucella spp. appear to lack most classical virulence factors, our work has shown that the virB locus, encoding a Type IV secretion system (T4SS), is essential for survival in mononuclear phagocytes and virulence in a mouse infection model. While the molecular mechanisms underlying T4SS-mediated intracellular persistence are unknown, we have recently identified (i) the first substrates of the T4SS, and (ii) a role for the T4SS in triggering granulomatous inflammation in vivo. Our long-range goal is to define molecular mechanisms of Brucella interaction with the host that lead to disease. The objective of this application is to pinpoint at the cellular level, how the T4SS interacts with innate immune signaling pathways to trigger a response that is favorable for colonization. The central hypothesis of this application is that the T4SS functions in establishing a niche in the host by eliciting caspase-1 dependent proinflammatory responses in infected cells. The central hypothesis will be tested and the objectives of this application accomplished by (1) Identifying the cytosolic sensors that detect intracellular Brucella infection and (2) Determining the role of secreted T4SS substrates in modulating the caspase-1 inflammasome. We expect that the results of this work will establish new paradigms of how the T4SS elicits cellular responses to promote establishment of Brucella in its host niche, thereby significantly influencing concepts and methods driving the field.